The invention pertains to a braking pressure generator for a hydraulic brake systen for automotive vehicles, with a master brake cylinder whose working chamber is in communication with an unpressurized compensating reservoir in the brake release position by way of a valve closable by the actuating stroke of the master cylinder piston. A hydraulic brake power booster is connected upstream of the master brake cylinder and comprises a booster piston designed as a stepped piston which latter penetrates the booster chamber and is composed of two pistons of differing effective surfaces movable in relation to one another. The pistons are applicable in opposed directions by the pressure in the booster chamber and stops limit their relative movement to an amount which at least corresponds approximately to the closure travel of the valve in the master brake cylinder. Both pistons are movable by resilient means to abut on one another in the direction of brake release, and the piston of smaller effective surface is movable to abut on a housing stop, as illustrated in patent application No. P 35 23 372.9.
In such braking pressure generators only little actuating force and low pressure in the booster chamber are required to initiate a braking action. This is accomplished in that, in the initial phase of braking, first the piston with the larger hydraulic effective surface is effective alone for the purpose of actuating the master brake cylinder so that the entire larger effective surface is available in order to generate the displacement force required to overcome the friction caused by the seals and the resetting springs. The piston of smaller effective surface remains in abutment on the housing stop in this initial phase of braking and thus contributes also to reducing the initial resistance to displacement. Furthermore, the moving apart of the two pistons in the initial phase of braking reduces the actuating travel at the control member of the braking pressure generator, and desired actuating characteristics are thereby achieved.
In the braking pressure generator according to the noted patent application, the two pistons of the booster piston will be kept apart exclusively by the pressure in the booster chamber. If the pressure in the booster chamber drops or if the braking pressure generator is actuated beyond its maximum boosting effect, the pistons will move towards one another again and thus bring about increase of the actuating travel at the control member of the braking pressure generator.
It is the object of the present invention to provide a braking pressure generator of the type referred to wherein the pistons of the booster piston are maintained in their position moved apart from one another during an actuating action even in the event of pressure drop in the booster chamber or in excess of the maximum boosting effect.